1. Technical Field of the Invention
The present invention relates to an image display apparatus, and particularly to an image display apparatus having a light emitting element in each of its pixels.
2. Prior Art
Among the image display apparatuses employing a light emitting element in each of its pixels, many reports have been made on EL displays using electroluminescent (hereinafter abbreviated as EL) elements.
In the active matrix type EL display, wiring lines for transmitting signals and currents are arranged in a matrix configuration, and a pixel circuit formed of thin film transistors (hereinafter abbreviated as TFTS), which are active elements, is incorporated in addition to the EL element within each of its pixels.
As methods for the pixel circuit to control light intensity of the EL element, there are a method by controlling a voltage supplied to the EL element by the pixel circuit, and another way by controlling a current supplied to the EL element by the pixel circuit.
The method by controlling the current provides the following advantages: (1) The control is facilitated because the light intensity of the EL element varies in proportion to the current; (2) The method is less susceptible to a voltage drop due to power supply lines; and (3) The method is not prone to deterioration of the EL element. A method of controlling light intensity of the EL element by controlling the current is reported in connection with FIGS. 7 and 8 at pages 875–878, IEEE, IEDM 98.
FIG. 14 illustrates a conventional pixel using an EL element. A pixel 150 is composed of a pixel circuit and an EL element 156. The pixel circuit is composed of TFT 151–TFT 154, and a capacitor 155. TFT 151 and TFT 153 are turned ON when an analog current IDADA, which is a display signal, is written into the pixel circuit, thereby the current IDATA flows into the EL element 156 via TFT 151 and TFT 152. The capacitor 155 stores a voltage V between gate and source electrodes of TFT 152 which is required for TFT 152 to flow the current IDATA therethrough. The stored current is reproduced in the EL element 156 by turning ON TFT 154 and thereby supplying the current to TFT 152. At this time, since the voltage V is stored in the capacitor 155, a current flowing through TFT 154, that is, a current flowing through the EL element 156, is controlled by the current IDATA. The light intensity of the EL element 156 is proportional to the current flowing therethrough, and therefore, the light intensity of the EL element can be controlled based upon the analog current IDADA, which is the display signal. Among EL elements varying its light intensity in proportion to a current therethrough, an organic EL diode is known. An image is displayed by writing currents IDATA successively into such pixels arranged in two dimensions.